1. Field of the Invention
The invention relates in general to superconducting transformers and in particular to current limiting superconducting transformers.
2. Description of the Prior Art
A modern large transformer is a highly efficient device with the resistive losses of the windings being considerably under one half percent of the total power transferred. However, this can represent a significant cost factor over the lifetime of the transformer. Accordingly, there has been increasing emphasis in recent years in replacing the copper windings of a conventional transformer with superconducting windings in order to reduce or eliminate these losses.
Both conventional and superconducting transformers are subject to fault or overload transient currents which can approach ten times the steady state current carried by the transformer. If the windings are designed to carry these currents, potential loss savings using superconducting windings is limited. This is because hysteresis loss in a superconducting wire that is subject to a time varying magnetic field is proportional to its volume. Thus, there would be no advantage to reducing resistive losses of a conventional transformer by replacing the copper windings with superconducting windings that are subject to hysteresis losses.
A significant breakthrough for reducing hysteresis losses in a superconducting transformer is disclosed in copending patent application Ser. No. 036,950, entitled "Superconducting Induction Apparatus", assigned to the same assignee as the present invention. That application discloses eliminating much of the hysteresis losses of a superconducting transformer winding by placing the bulk of the superconducting windings in parallel auxiliary windings and locating these auxiliary windings in field-free regions. Copending application Ser. No. 036,950 also discloses a way of spacing the auxiliary windings relative to the main windings such that they are characterized by a higher impedance than the main windings and consequently negligible current flows in them during steady state operation. The beneficial result of this invention is that the auxiliary windings are not subject to hysteresis losses in the absence of fault or overload conditions because they are substantially non-conducting and located in field-free regions.
Copending application Ser. No. 036,950's design concepts resulted in a superconducting transformer that can carry fault currents of ten times the steady state currents while still achieving low losses under steady state conditions. This design, while a significant improvement over the prior art still required a superconducting winding (whether one winding or divided into main and auxiliary windings) sized ten times that which is necessary to operate under steady state conditions in order to withstand infrequent transient current excursions. Also required, as will be more fully developed infra, is large spaces between windings, necessary both to locate the auxiliary windings in field-free regions and to provide for sufficient insulation to electrically insulate adjacent windings from one another. This last requirement becomes critically important in the higher voltage transformers where a conversion to superconducting windings is economically feasible.
Accordingly, it would be desirable to design a superconducting transformer that would automatically change from a lower input impedance to a higher input impedance when beset by transient and fault currents and thereby inherently limit the magnitude of the fault currents themselves. This would reduce the winding size requirements of the transformer and associated electrical apparatus. It would also be desirable to design an arrangement of primary and secondary main and auxiliary windings so as to minimize the spacing requirements between adjacent windings.